Strengthened thin glass-polymer laminates

ABSTRACT

A glass-polymer laminate structure includes a flexible glass substrate having a thickness of no more than about 0.3 mm. A polymer layer is laminated to a surface of the flexible glass substrate having a coefficient of thermal expansion (CTE) that is at least about 2 times a CTE of the flexible glass substrate. The polymer layer is laminated to the surface of the flexible glass substrate after thermally expanding the polymer layer to provide the flexible glass substrate with an in-plane compressive stress of at least about 30 MPa along a thickness of the flexible glass substrate.

This application is a divisional of U.S. patent application Ser. No.14/423,956 filed on Feb. 25, 2015, which claims the benefit of priorityunder 35 U.S.C. § 371 of International Application No.PCT/US2013/056755, filed on Aug. 27, 2013, which claims the benefit ofpriority under 35 U.S.C. § 119 of U.S. Provisional Application Ser. No.61/695,781 filed on Aug. 31, 2012 the content of each of which is reliedupon and incorporated herein by reference in its entirety.

FIELD

The present invention relates to glass-polymer laminate structures and,more particularly, to strengthened thin glass-polymer laminatestructures.

BACKGROUND

Flexible polymer substrates are manufactured using a polymer basematerial laminated with one or more polymer films. These laminatedsubstrate stacks are commonly used in flexible packaging associated withPV, OLED, LCDs and patterned Thin Film Transistor (TFT) electronicsbecause of their low cost and demonstrated performance.

In order to promote flexible glass structures as an alternate technologyselection, the real and perceived limitations of mechanical reliabilityperformance associated with glass, a brittle material, must be overcomeand demonstrated. Flexible glass substrates offer several technicaladvantages over flexible polymer technology. One technical advantage isthe ability of the glass to serve as a moisture or gas barrier, aprimary degradation mechanism in outdoor electronics. A second advantageis in its potential to reduce overall package size (thickness) andweight through the reduction or elimination of one or more packagesubstrate layers. Another advantage is having excellent surfacequalities associated with glass that can be cleaned easily. Thus, if thereal and perceived limitations of mechanical reliability performanceassociated with glass can be overcome, the use of flexible glassstructures can be advanced.

SUMMARY

One technique to improve the mechanical reliability of bare flexibleglass is to laminate the flexible glass substrate with one or more thinfilm polymers. Depending on the mechanical strength requirements and theexpected bending stresses and direction of the end application,according to the concepts disclosed herein, a flexible glass-polymerlaminate substrate can be designed to meet mechanical requirements. Whenused properly, the laminated structure will offer improved mechanicalreliability performance over a flexible un-laminated (bare glass)structure.

Additional features and advantages will be set forth in the detaileddescription which follows, and in part will be readily apparent to thoseskilled in the art from the description or recognized by practicing theinvention as exemplified in the written description and the appendeddrawings. It is to be understood that both the foregoing generaldescription and the following detailed description are merely exemplaryof the invention, and are intended to provide an overview or frameworkto understanding the nature and character of the invention as it isclaimed.

The accompanying drawings are included to provide a furtherunderstanding of principles of the invention, and are incorporated inand constitute a part of this specification. The drawings illustrate oneor more embodiment(s), and together with the description serve toexplain, by way of example, principles and operation of the invention.It is to be understood that various features of the invention disclosedin this specification and in the drawings can be used in any and allcombinations. By way of non-limiting example the various features of theinvention may be combined with one another according to the followingaspects.

According to a first aspect, there is provided a glass-polymer laminatestructure comprising:

a flexible glass substrate having a thickness of no more than about 0.3mm; and

a polymer layer laminated to a surface of the flexible glass substratehaving a coefficient of thermal expansion (CTE) that is at least about 2times a CTE of the flexible glass substrate, the polymer layer laminatedto the surface of the flexible glass substrate after thermally expandingthe polymer layer to provide the flexible glass substrate with acompressive stress of at least about 30 MPa across a thickness of theflexible glass substrate.

According to a second aspect, a glass-polymer laminate structurecomprising:

a flexible glass substrate having a thickness of no more than about 0.3mm; and

a polymer layer laminated to a surface of the flexible glass substratehaving a coefficient of thermal expansion (CTE) that is at least about 3ppm/° C. greater than a CTE of the flexible glass substrate, the polymerlayer laminated to the surface of the flexible glass substrate afterthermally expanding the polymer layer to provide the flexible glasssubstrate with a compressive stress of at least about 30 MPa across athickness of the flexible glass substrate.

According to a third aspect, there is provided the laminate structure ofaspect 1 or aspect 2, wherein the flexible glass substrate has anin-plane compressive stress of at least about 80 MPa across thethickness of the flexible glass substrate.

According to a fourth aspect, there is provided the laminate of any oneof aspects 1 to 3, wherein the polymer layer has a CTE that is at leastabout 10 times the CTE of the flexible glass substrate.

According to a fifth aspect, there is provided the laminate of any oneof aspects 1 to 4, further comprising an adhesive layer that laminatesthe polymer layer to the flexible glass substrate.

According to a sixth aspect, there is provided the laminate of aspect 5,wherein the adhesive layer is heat activated, having an activationtemperature of greater than about 50° C.

According to a seventh aspect, there is provided the laminate of aspect5, wherein the adhesive layer is a pressure sensitive adhesive.

According to an eighth aspect, there is provided the laminate of aspect5, wherein the adhesive layer is UV activated.

According to a ninth aspect, there is provided the laminate of aspect 1to 8, wherein the flexible glass substrate is a first flexible glasssubstrate, the laminate structure further comprising a second flexibleglass substrate laminated to the polymer layer, the polymer layer beingbetween the first and second flexible glass substrates.

According to a tenth aspect, there is provided the laminate of aspect 9,wherein the polymer layer is formed of a liquid polymer.

According to an eleventh aspect, there is provided the laminate ofaspect 10, wherein the polymer layer extends beyond an outer edge of atleast one of the first and second flexible glass substrates.

According to a twelfth aspect, there is provided the laminate of aspect1 to 11, wherein the polymer layer is a first polymer layer, thelaminate structure further comprising a second polymer layer laminatedto the flexible glass substrate, the flexible glass substrate beingbetween the first and second polymer layers.

According to a thirteenth aspect, there is provided a method of forminga flexible glass-polymer laminate structure, the method comprising:

heating a polymer layer of the flexible glass-polymer laminate structureto an elevated temperature of greater than 20° C., the polymer layerhaving a coefficient of thermal expansion (CTE) that is at least 2 timesa CTE of a flexible glass substrate of the flexible glass-polymerlaminate structure;

laminating the polymer layer at the elevated temperature to the flexibleglass substrate; and

cooling the polymer layer below the elevated temperature to introduce acompressive stress of at least about 30 MPa across a thickness of theflexible glass substrate.

According to a fourteenth aspect, a method of forming a flexibleglass-polymer laminate structure, comprising:

heating a polymer layer of the flexible glass-polymer laminate structureto an elevated temperature of greater than 20° C., the polymer layerhaving a coefficient of thermal expansion (CTE) that is at least about 3ppm/° C. greater than a CTE of a flexible glass substrate of theflexible glass-polymer laminate structure;

laminating the polymer layer at the elevated temperature to the flexibleglass substrate; and

cooling the polymer layer below the elevated temperature to introduce acompressive stress of at least about 30 MPa across a thickness of theflexible glass substrate.

According to a fifteenth aspect, there is provided the method of aspect12 or aspect 13, wherein the flexible glass substrate has an in-planecompressive stress of at least about 80 MPa across the thickness of theflexible glass substrate.

According to a sixteenth aspect, there is provided the method of aspect13 to 15, wherein the polymer layer has a CTE that is at least about 10times the CTE of the flexible glass substrate.

According to a seventeenth aspect, there is provided the method ofaspect 11 to 13, comprising expanding the polymer layer relative to theflexible glass substrate as the polymer layer is heated to the elevatedtemperature.

According to an eighteenth aspect, there is provided the method ofaspect 13 to 17, further comprising applying an adhesive layer betweenthe flexible glass substrate and the polymer layer that laminates thepolymer layer to the flexible glass substrate.

According to a nineteenth aspect, there is provided the method of aspect18, comprising heat activating the adhesive layer after the polymerlayer reaches the elevated temperature.

According to a twentieth aspect, there is provided the method of aspect18, comprising pressure activating the adhesive layer after the polymerlayer reaches the elevated temperature.

According to a twenty-first aspect, there is provided the method ofaspect 18, comprising UV activating the adhesive layer after the polymerlayer reaches the elevated temperature.

According to a twenty-second aspect, there is provided a method offorming a flexible glass-polymer laminate structure, the methodcomprising:

heating a polymer layer of the flexible glass-polymer laminate structureto an elevated temperature of greater than 20° C., the polymer layerhaving a coefficient of thermal expansion (CTE) that is at least about 3ppm/° C. greater than a CTE of a flexible glass substrate of theflexible glass-polymer laminate structure;

laminating the polymer layer at the elevated temperature to the flexibleglass substrate; and

cooling the polymer layer below the elevated temperature to bend theflexible glass substrate while simultaneously providing a compressivestress across a thickness of the flexible glass substrate.

According to a twenty-third aspect, there is provided the method ofaspect 22, wherein the step of heating the polymer layer comprisesheating the polymer layer to a temperature of at least about 50° C.before laminating the polymer layer to the flexible glass substrate.

According to a twenty-fourth aspect, a flexible glass-polymer laminatestructure, comprising:

a flexible glass substrate having a thickness of no more than about 0.3mm; and

a polymer layer laminated to a surface of the flexible glass substratehaving a coefficient of thermal expansion (CTE) that is at least about 3ppm/° C. greater than a CTE of the flexible glass substrate, the polymerlayer laminated to the surface of the flexible glass substrate afterthermally expanding the polymer layer providing the flexible glasssubstrate with a bent configuration while simultaneously providing acompressive stress along at least a portion of a thickness of theflexible glass.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustration of an embodiment of a symmetricflexible glass-polymer laminate structure;

FIG. 2 is a schematic illustration of another embodiment of a symmetricflexible glass-polymer laminate structure;

FIG. 3 is a schematic illustration of another embodiment of a flexibleglass-polymer laminate structure having edge protection features;

FIG. 4 illustrates schematically another embodiment of a flexibleglass-polymer laminate structure having an asymmetric configuration;

FIG. 5 is a schematic drawing of a piece of glass having a neutralbending axis;

FIG. 6 illustrates a tensile stress reduction in an exemplary thermallybent laminate;

FIG. 7 is a schematic illustration of another embodiment of a flexibleglass-polymer laminate structure; and

FIG. 8 is a schematic illustration of another embodiment of a flexibleglass-polymer laminate structure.

DETAILED DESCRIPTION

In the following detailed description, for purposes of explanation andnot limitation, example embodiments disclosing specific details are setforth to provide a thorough understanding of various principles of thepresent invention. However, it will be apparent to one having ordinaryskill in the art, having had the benefit of the present disclosure, thatthe present invention may be practiced in other embodiments that departfrom the specific details disclosed herein. Moreover, descriptions ofwell-known devices, methods and materials may be omitted so as not toobscure the description of various principles of the present invention.Finally, wherever applicable, like reference numerals refer to likeelements.

Ranges can be expressed herein as from “about” one particular value,and/or to “about” another particular value. When such a range isexpressed, another embodiment includes from the one particular valueand/or to the other particular value. Similarly, when values areexpressed as approximations, by use of the antecedent “about,” it willbe understood that the particular value forms another embodiment. Itwill be further understood that the endpoints of each of the ranges aresignificant both in relation to the other endpoint, and independently ofthe other endpoint.

Directional terms as used herein—for example up, down, right, left,front, back, top, bottom—are made only with reference to the figures asdrawn and are not intended to imply absolute orientation.

Although glass is an inherently strong material, its strength(reliability) is a function of its surface defect or flaw size densitydistribution and the cumulative exposure of stress to the material overtime. During the entire product life cycle, a flexible glass-polymerlaminate will be subjected to various kinds of static and dynamicmechanical stresses. Embodiments described herein generally relate toflexible glass substrates that are strengthened using a polymer layer. Arelatively large coefficient of thermal expansion (CTE) mismatch betweenthe polymer layer and the flexible glass substrate is utilized bylaminating the polymer layer to the flexible glass substrate at anelevated temperature followed by a slow cooling. Such an elevatedtemperature lamination approach can create a uniformly distributedcompressive residual stress across the thickness of the flexible glasssubstrate once the laminate structure is cooled.

Referring to FIGS. 1 and 2, two exemplary flexible glass-polymerlaminate structures 10 and 40 are illustrated along with theircorresponding stress diagrams 12 and 42. Referring first to FIG. 1, theflexible glass-polymer laminate structure 10 includes a first outermostglass layer 14 that is formed by a first flexible glass substrate 16(having Eg, Vg, α, t_(g)/2), a second outermost glass layer 18 that isformed by a second flexible glass substrate 20 (having Eg, Vg, α,t_(g)/2) and a polymer layer 22 (having Ep, Vp, α_(p), t_(p)) that issandwiched between and laminated to the first and second flexible glasssubstrates 16 and 20. Adhesive layers 24 and 26 may be used to laminatethe first and second flexible glass substrates 16 and 20 to the polymerlayer 22 at the interfaces between their respective broad surfaces 28,30 and 32, 34.

While FIG. 1 illustrates outermost glass layers 14 and 18, FIG. 2illustrates an alternative embodiment with a flexible glass substrate 44(having Eg, Vg, α, t_(g)) that is sandwiched between outermost polymerlayers 50 and 52 (each having Ep, Vp, α_(p), t_(p)/2). Again, adhesivelayers 54 and 56 may be used to laminate the first and second polymerlayers 50 and 52 to the flexible glass substrate 44 at the interfacesbetween their respective broad surfaces 58, 60 and 62, 64.

The stress diagrams 12 and 42 of FIGS. 1 and 2 illustrate that residual,in-plane compressive stresses are generated across the thicknesses ofthe flexible glass substrates 16 and 20 of the flexible glass-polymerlaminate structure 10 and the flexible glass substrate 44 of flexibleglass-polymer laminate structure 40, which are compensated by tensilestresses in the polymer layer 22 of the flexible glass-polymer laminatestructure 10 and polymer layers 50 and 52 of the flexible glass-polymerlaminate structure 40. Referring first to FIG. 1, the residualcompressive stresses in the flexible glass substrates 16 and 20 of theflexible glass-polymer laminate structure 10 can be substantiallyuniformly distributed across the thicknesses of the flexible glasssubstrates 16 and 20 and generated by laminating the polymer layer 22 tothe flexible glass substrates 16 and 20 at an elevated temperature andthen cooling to room temperature. Similarly, referring to FIG. 2, theresidual compressive stresses in the flexible glass substrate 44 of theflexible glass-polymer laminate structure 40 can be substantiallyuniformly distributed across the thickness of the flexible glasssubstrate 44 and generated by laminating the polymer layers 50 and 52 tothe flexible glass substrate 44 at an elevated temperature and thencooling to room temperature, which will be described in greater detailbelow. Further, while tri-layer laminate structures are illustrated byFIGS. 1 and 2, the number of layers can be greater or less than threelayers and selected depending on, for example, the end use andprocessing requirements. Various other layered laminate examples will bedescribed herein.

The flexible glass substrates 16, 20 and 44 may be “ultra-thin” having athickness t_(g) of about 0.3 mm or less including but not limited tothicknesses of, for example, about 0.01-0.05 mm, about 0.05-0.1 mm,about 0.1-0.15 mm and about 0.15-0.3 mm. The flexible glass substrates16, 20 and 44 may be formed of glass, a glass ceramic, a ceramicmaterial or composites thereof. A fusion process (e.g., downdrawprocess) that forms high quality thin glass sheets can be used in avariety of devices such as flat panel displays. Glass sheets produced ina fusion process have surfaces with superior flatness and smoothnesswhen compared to glass sheets produced by other methods. The fusionprocess is described in U.S. Pat. Nos. 3,338,696 and 3,682,609. Othersuitable glass sheet forming methods include a float process, updraw andslot draw methods.

Strengthening Symmetric Flexible Glass-Polymer Laminates

A process is developed for strengthening flexible glass substrates thatutilizes a large CTE mismatch (e.g., about 2 times or more, such asabout 5 times or more, such as about 10 times or more) between thepolymer layer and the flexible glass substrate by laminating the polymerlayer and the flexible glass substrate (e.g., as shown in FIGS. 1 and 2)at an elevated temperature and then slowly cooling to create a residualcompressive stress across the thickness of the flexible glass substrate.In some embodiments, the CTE mismatch may be at least about 3 ppm/° C.or more, such as about 6 ppm/° C. or more, such as about 9 ppm/° C. ormore, such as about 12 ppm/° C. or more, such as about 15 ppm/° C. ormore, such as about 20 ppm/° C. or more, such as about 27 ppm/° C. ormore, such as about 50 ppm/° C. or more. The flexible glass-polymerlaminates may be classified as symmetric and asymmetric. A symmetriclaminate structure is constructed such that the layers below a centralplane C (FIGS. 1 and 2) of the laminate structure form a mirror image ofthe layers above the central plane C and asymmetric laminates do nothave such a mirror image about their central planes. For symmetricflexible glass-polymer laminates formed of two different materials andhaving three or more layers with different CTE laminated at an elevatedtemperature, the compressive stress across the thickness of the flexibleglass substrate at room temperature (assuming bi-axial deformation) isgiven by:

$\sigma_{g} = {{- \frac{E_{g}}{\left( {1 - v_{g}} \right) + {E_{g}{{t_{g}\left( {1 - v_{p}} \right)}/E_{p}}t_{p}}}}\left( {\alpha_{p} - \alpha_{g}} \right)\left( {T_{lamination} - T_{room}} \right)}$

wherein:

-   -   E is Young's modulus, a is linear thermal expansion coefficient,        t is total thicknesses of one type of material, v is Poisson's        ratio, and the subscripts “g” and “p” refer to “glass” and        “polymer”, respectively. T_(lamination) refers to the curing        temperature of the adhesive used in the lamination process.        Thus, the compressive stress in the flexible glass substrate can        be increased by one or more of    -   1. reducing the glass layer thickness;    -   2. increasing the Young's modulus of the flexible glass        substrate;    -   3. increasing the Young's modulus of the polymer layer;    -   4. increasing the thickness polymer layer;    -   5. increasing the thermal expansion coefficient difference        between the flexible glass substrate and the polymer layer; and    -   6. increasing the lamination temperature.

While larger compressive stresses (e.g., 30 MPa or more, such as 50 MPaor more, such as 60 MPA or more, such as about 70 MPa or more such asabout 80 MPa or more, such as about 90 MPA or more, such as about 100MPa or more, such as about 110 MPa or more) are desired across thethickness of the flexible glass substrates, there are limits on theamount of compressive stress that can be introduced. For example, toachieve large compressive stresses in the flexible glass substrate, oneapproach is to set the lamination temperature as high as possible.However, the upper end of this lamination temperature should not exceedlimits set by specific properties of the flexible glass-polymer laminatematerials, such as the working temperature limit of any adhesive used.Material and structural integrity should be considered in insuringproduct reliability. Thus, various limits may affect the amount ofcompressive stress that can be introduced to the flexible glasssubstrate.

Forming Strengthened Flexible Glass-Polymer Laminates

Any number of lamination processes may be used to achieve the desiredhigh compressive stresses in the flexible glass substrates. Importantly,the polymer layer should be allowed to thermally expand, at least tosome degree, relative to the flexible glass substrate due to the largeCTE mismatch between the flexible glass substrate and the polymer layerbefore laminating the polymer layer to the flexible glass substrate.Once laminated, the flexible glass-polymer laminate structure may beallowed to controllably cool (e.g., about 1-2° C./min or less) back downto room temperature, which introduces the compressive stress into theflexible glass substrate.

Examples of laminating at an increased temperature for generating acompressive stress due to a large CTE mismatch between a polymer layerand a flexible glass substrate are described below. These examples areexemplary in nature and are not meant to be limiting. For example, whilea polymer layer of polymethyl methacrylate (PMMA) may be used in many ofthe examples discussed below, a polymer layer of polycarbonate may beused, as well as a number of other polymer materials, which arediscussed below, having a relatively large CTE mismatch with theflexible glass substrate. PMMA and polycarbonate polymer materials maybe desirable where transparency in the end use of the flexibleglass-polymer laminate is preferred.

Example 1: UV Sensitive Adhesive

A flexible glass-polymer laminate structure as shown in FIG. 1 wasformed using two flexible glass substrates 0.1 mm in thickness and apolymer layer formed of polymethyl methacrylate (PMMA), a transparentthermoplastic, having a thickness of 1.3 mm. A UV sensitive adhesive(NOA 68 commercially available from Norland Products) was applied about20 nm in thickness between the polymer layer and the flexible glasssubstrate layers. The flexible glass-polymer laminate structure wasplaced on a thermoelectric hotplate and heated from one side maintainedat 80° C., which was below the UV sensitive adhesive's working limit of90° C. The UV sensitive adhesive was then cured using UV light appliedto the UV sensitive adhesive from a side of the flexible glass-polymerlaminate structure opposite the thermoelectric hotplate to bond thepolymer layer at an elevated temperature to the flexible glass substratelayers and allowed to cool at a rate of 3° C./min. About a 110 MPacompressive stress in the flexible glass substrate was confirmed usingbirefringence measurements (FSM). Table I below highlights materialproperties for this exemplary flexible glass-polymer laminate structureand Table II below illustrates stress estimates using the compressivestress formula set forth above.

TABLE I Materials Coefficient Lamination Young's of Thermal Temp/Modulus Poisson's Expansion Thickness Room Material (GPa) Ratio (10⁻⁶/°C.) (mm) Temp (° C.) PMMA 2.5 0.37 90 1.3 80/20 Glass 73.6 0.23 3.17 0.1(0.2 Substrate total)

TABLE II Stress Estimates Compressive Residual Stress Tensile ResidualStress in in Glass Substrate (MPa) PMMA (MPa) Estimate 105.8 16.3

Example 2: Heat Sensitive Adhesive

A flexible glass-polymer laminate structure as shown in FIG. 1 wasformed using two flexible glass substrates 0.1 mm in thickness and apolymer layer formed of PMMA having a thickness of 1.3 mm. A heatsensitive adhesive (NOA 68H commercially available from NorlandProducts) was applied about 20 nm in thickness between the polymer layerand the flexible glass substrate layers. The flexible glass-polymerlaminate structure was heated in a convection oven to a laminationtemperature (cure at 100° C. for 3 hours, 3° C./min ramp-up rate, 1°C./min cooling rate, aged at 50° C. for 12 hours before cooling down toroom temperature), which was below the softening temperature of the PMMA(91° C. to 115° C.), allowing the polymer layer to expand as thelamination temperature was reached to bond the polymer layer at anelevated temperature to the flexible glass substrate layers.

Example 3: Pressure Sensitive Adhesive

A flexible glass-polymer laminate structure as shown in FIG. 1 wasformed using two flexible glass substrates 0.1 mm in thickness and apolymer layer formed of PMMA having a thickness of 1.3 mm. A pressuresensitive adhesive (8211 OCA commercially available from 3M) was applied(about 50 μm thick) between the polymer layer and the flexible glasssubstrate layers. The flexible glass-polymer laminate structure washeated in an autoclave to a lamination temperature, which was below thesoftening temperature of the PMMA (91° C. to 115° C.), allowing thepolymer layer to expand as the lamination temperature was reached tobond the polymer layer at an elevated temperature to the flexible glasssubstrate layers (cured at 90 psi and 85° C. for 1 hour, 3° F./min rampup and cooling rate).

While the above examples utilize an intermediate adhesive layer to bondthe polymer layer and flexible glass substrate, other embodiments mayinclude the polymer layer bonded directly to the flexible glasssubstrate without any use of an intermediate adhesive layer. Forexample, a polymer layer may be heated to a temperature above theplastic softening temperature (glass transition temperature), but belowthe melting temperature for the polymer. For PMMA, for example, thesoftening temperature is 91° C. to 115° C. and the melting temperatureis 160° C. A combination of heat and pressure may be utilized (e.g.,using an autoclave) to heat the polymer layer to a temperature betweenthe softening temperature and the melting temperature. In someinstances, the heated temperature may be held for a preselected periodof time and then the flexible glass-polymer laminate structure may becooled at a predetermined rate (e.g., less than about 3° F./min).

As another example of a polymer layer, a liquid polymer solution may beinjected into a space between flexible glass substrates. Referringbriefly to FIG. 3, at an elevated temperature, a flexible glass-polymerlaminate 70 may be formed by separating flexible glass substrates 72 and74, for example, using spacers (represented by dotted lines 76). Theliquid polymer solution 78 may be injected into a space 80 formedbetween the flexible glass substrates 72 and 74. Lamination of thepolymer layer 82 to the flexible glass substrates 72 and 74 may becompleted through polymer solidification (curing for example) and acontrolled cooling step. Capillary forces, viscous flow and expansion ofthe liquid polymer solution 78 during heating may force the liquidpolymer solution 78 beyond outer edges 84 and 86 of the flexible glasssubstrates 72 and 74, which can lead to encapsulating and covering theouter edges 84 and 86. This encapsulation of the outer edges 84 and 86can provide an edge protection feature 88 for the flexible glasssubstrates 72 and 74 once the polymer layer 82 is formed. Such edgeprotection can be advantageous as the compressive stress at the edges ofthe flexible substrates 72 and 74 may be relatively low or even notpresent, even after formation of the polymer layer 82 and the edges mayhave a relatively low strength compared to the bulk glass. To facilitatebonding, adhesive promoters may be added to the polymer solution 78.Physical bonding, and in some instances chemical bonding, occurs betweenthe polymer layer 82 and the flexible glass substrates 72 and 74.

In addition to the above-mentioned heaters and ovens, non-contactheaters, such as radiative heaters (and microwave heating) can be usedto heat the polymer layers discussed herein. Radiative heaters emitinfrared radiation, which can be absorbed by materials resulting in heattransfer to the heated body. Radiative heating can be efficient andrapid compared to convective or conductive heating, and does not requirecontact with the surface of the heated body. As one alternative to theheating of Example 1, the flexible glass-polymer laminate structure maybe passed between a UV light source and a radiative heater, such that UVcuring of the adhesive material is obtained at the desired temperature,which can be maintained by the radiative heating. Further, as avariation of Example 2, the convection oven may be replaced by tworadiative heaters, one on either side of the flexible glass-polymerlaminate structure. The process can be continuous by controlling thefeed rate and power of the heaters to achieve heating, the laminationtemperature and cooling. Multiple radiative heaters may be used tocontrol heating, dwell and cooling rates.

Asymmetric Flexible Glass-Polymer Laminates

Lamination processes may also be used to manipulate or affect stressprofiles in asymmetric flexible glass-polymer laminate structures. Inthese embodiments, the polymer layer is allowed to thermally expandrelative to the flexible glass substrate due to the large CTE mismatchbetween the flexible glass substrate and the polymer layer beforelaminating the polymer layer to the flexible glass substrate. However,due to the asymmetric nature of the flexible glass-polymer laminatestructures, bending, uniaxial or biaxial, may be introduced in theflexible glass substrate and the polymer layer.

Referring to FIG. 4, a uniaxial bending curvature induced by a CTEmismatch in an asymmetric flexible glass-polymer laminate structure 90is given by (assuming plane stress and the flexible glass substrate 96and polymer layer 98 have the same width and length dimensions):

$\kappa = \frac{6\left( {\alpha_{p} - \alpha_{g}} \right)\left( {T_{laminaton} - T_{room}} \right)t_{g}{t_{p}\left( {t_{g} + t_{p}} \right)}}{{E_{g}{t_{g}^{4}/E_{p}}} + {4t_{g}^{3}t_{p}} + {6t_{g}^{2}t_{p}^{2}} + {4t_{g}t_{p}^{3}} + {E_{p}{t_{p}^{4}/E_{g}}}}$

wherein:

-   -   κ is bending curvature, E is Young's modulus, a is linear        thermal expansion coefficient, t is total thicknesses of one        type of material, and the subscripts “g” and “p” refer to        “glass” and “polymer”, respectively. T_(lamination) refers to        the curing temperature of the adhesive used in the lamination        process.        Bending curvature κ is related to the radius R measured from        center C to the neutral axis A by:

R=1/κ

and is also related to an angle θ measured between ends 92 and 94 of theflexible glass-polymer laminate structure 90 by:

θ=L/R=κL.

Height h to the neutral axis can be determined by:

${h = {{R\left( {1 - {\cos \left( \frac{\theta}{2} \right)}} \right)} = {\frac{1}{\kappa}\left( {1 - {\cos \left( \frac{\kappa L}{2} \right)}} \right)}}}.$

Thus, bending characteristics of an asymmetric flexible glass-polymerlaminate structure can be determined where the material properties andlamination temperatures are known. For example, for a 100 μm/500 μm/100μm glass/polycarbonate/glass laminate, where polycarbonate has a Young'smodulus E of 2.377 GPa, a CTE a of 67.5×10⁻⁶/° C. and a laminationtemperature T_(lamination) of 80° C., the height h, once cooled to roomtemperature, is about 11.3 mm.

Referring to FIG. 5, when an uncoated piece of glass 100 (either a sheetor ribbon of glass) is bent into a curve of radius R (such as when theglass is held in a device or bends around a roller during processing),there is created a stress in the glass. When the piece of glass 100 issubject to a bending moment, so that the glass achieves a bending radiusR, the stress at a location (y) relative to the x-axis (neutral axis)can be calculated by:

$\sigma = {\frac{E_{g}y}{R\left( {1 - \nu_{g}^{2}} \right)}\left( {{plain}\mspace{14mu} {strain}} \right)}$or$\sigma = {\frac{E_{g}y}{R}\left( {{plain}\mspace{14mu} {stress}} \right)}$

-   -   wherein:    -   σ is the stress;    -   E_(g) is the Young's modulus of the glass;    -   v_(g) is the Poisson's ratio of the glass;    -   y is the position in the y-axis direction at which the stress 6        is calculated;    -   R is the bending radius of the glass.

The maximum tensile stress occurs on either side of the glass 100, andwill be either surface 102 or surface 104 depending on the bendingdirection. That is, if the glass 100 is bent so that surface 102 isconvex, the maximum tensile stress will be on surface 102, whereas ifthe glass 100 is bent so that surface 104 is convex, the maximum tensilestress will be on surface 104. In either case, by substituting ½ tg fory, the absolute value of the maximum stress σ_(max) is defined by:

${\sigma_{\max}} = {\frac{E_{g}t_{g}}{2{R\left( {1 - \nu_{g}^{2}} \right)}}\left( {{plain}\mspace{14mu} {strain}} \right)}$or${\sigma_{\max}} = {\frac{E_{g}t_{g}}{2R}\left( {{plain}\mspace{14mu} {stress}} \right)}$

Laminating the polymer layer 98 at an elevated temperature to theflexible glass substrate 96 as shown in FIG. 4, for example, allows formanipulation of the stress profile, compared to the uncoated glass ofFIG. 5. In particular, laminating the polymer layer 98 having the highCTE mismatch with the flexible glass substrate 96 to cause bending andintroduce compressive stress in the flexible glass substrate can reduceor even eliminate the tensile stress that would be expected in a bentglass and can serve to strengthen the overall bent flexible glasslaminate structure 90 (FIG. 4).

The following discussion illustrates the reduction in tensile stress inthe flexible glass substrate in the flexible-glass polymer laminate. Foran asymmetric flexible glass-polymer laminate formed of two differentmaterials, the curvature induced by CTE mismatch (assuming uniaxialplane stress bending) is (as noted above):

$\kappa_{0} = \frac{6\left( {\alpha_{p} - \alpha_{g}} \right)\left( {T_{lamination} - T_{room}} \right)t_{g}{t_{p}\left( {t_{g} + t_{p}} \right)}}{{E_{g}{t_{g}^{4}/E_{p}}} + {4t_{g}^{3}t_{p}} + {6t_{g}^{2}t_{p}^{2}} + {4t_{g}t_{p}^{3}} + {E_{p}{t_{p}^{4}/E_{g}}}}$

-   -   the neutral axis is given by:

$y_{0} = \frac{{{\alpha_{p}\left( {{E_{p}{t_{p}^{4}/E_{g}}} + {6t_{p}^{3}t_{g}} + {9t_{p}^{2}t_{g}^{2}} + {4t_{p}t_{g}^{3}}} \right)} + {\alpha_{g}\left( {{E_{g}{t_{g}^{4}/E_{p}}} - {3t_{g}^{2}t_{p}^{2}} - {2t_{g}t_{p}^{3}}} \right)}}\quad}{6\left( {\alpha_{p} - \alpha_{g}} \right)t_{g}{t_{p}\left( {t_{g} + t_{p}} \right)}}$

and the bending stress across the thickness of the flexible glasssubstrate at room temperature is given by:

σ=E _(g)(κ₀(y−y ₀)+α_(g)(T _(lamination) −T _(room)))

The bending stress at the top surface of the flexible glass substrate isgiven by:

$\sigma_{{glass}\; \_ \; {top}} = \frac{\left( {\alpha_{p} - \alpha_{g}} \right)\left( {T_{lamination} - T_{room}} \right){t_{p}\left( {{2E_{g}t_{g}^{3}} + {3E_{g}t_{g}^{2}t_{p}} - {E_{p}t_{p}^{3}}} \right)}}{{E_{g}{t_{g}^{4}/E_{p}}} + {4t_{g}^{3}t_{p}} + {6t_{g}^{2}t_{p}^{2}} + {4t_{g}t_{p}^{3}} + {E_{p}{t_{p}^{4}/E_{g}}}}$

-   -   and the bending stress at the bottom surface (i.e., the        interfacial surface with the polymer layer) is given by:

$\sigma_{{glass}\; \_ \; {bottom}} = {- \frac{\left( {\alpha_{p} - \alpha_{g}} \right)\left( {T_{lamination} - T_{room}} \right){t_{p}\left( {{4E_{g}t_{g}^{3}} + {3E_{g}t_{g}^{2}t_{p}} + {E_{p}t_{p}^{3}}} \right)}}{{E_{g}{t_{g}^{4}/E_{p}}} + {4t_{g}^{3}t_{p}} + {6t_{g}^{2}t_{p}^{2}} + {4t_{g}t_{p}^{3}} + {E_{p}{t_{p}^{4}/E_{g}}}}}$

-   -   wherein:    -   E_(g), V_(g) are the Young's modulus and Poisson's ratio of the        flexible glass substrate;    -   E_(p), v_(p) are the Young's modulus and Poisson's ratio of the        polymer layer;    -   t_(g) is the thickness of the flexible glass substrate;    -   t_(p) is the thickness of the plastic layer;    -   α_(g) is the coefficient of thermal expansion of the flexible        glass substrate;    -   α_(p) is the coefficient of thermal expansion of the polymer        layer;    -   T_(lamination) is the lamination temperature;    -   T_(room) is room temperature;    -   κ₀ is the bending curvature induced by the CTE mismatch;    -   σ is the bending stress; and    -   y₀ is the neutral axis of the bent laminates.

Referring to FIG. 6 and the above equations particularly for bendingstress, it can be seen that tensile stress inside a thermally bent,flexible glass substrate is much less than a mechanically bent flexibleglass substrate. This reduction in the tensile stress can be seenthroughout the thickness of the flexible glass substrate from the bottomside to the top side. It should be noted, however, that the stressacross the entire thickness of the flexible glass substrate is notuniform as depicted in FIGS. 1 and 2 due to the bending of the flexibleglass substrate. In this embodiment, only a portion of the thickness ofthe flexible glass is at or above 30 MPa nearer the bottom side of theflexible glass substrate.

General Considerations

The polymer layers for use in the laminate structures described hereinmay include various polymers, for example, any one or more ofpolyethylene teraphthalate (PET), polyethylene Naphthalate (PEN),ethylene tetrafluoroethylene (ETFE), or thermopolymer polyolefin(TPO™—polymer/filler blends of polyethylene, polypropylene, blockcopolymer polypropylene (BCPP), or rubber), polyesters, polycarbonate,polyvinylbuterate, polyvinyl chloride, polyethylene and substitutedpolyethylenes, polyhydroxybutyrates, polyhydroxyvinylbutyrates,polyetherimides, polyamides, polyethylenenaphalate, polyimides,polyethers, polysulphones, polyvinylacetylenes, transparentthermoplastics, transparent polybutadienes, polycyanoacrylates,cellulose-based polymers, polyacrylates and polymethacrylates,polyvinylalcohol, polysulphides, polyvinyl butyral, polymethylmethacrylate and polysiloxanes. It is also possible to use polymerswhich can be deposited/coated as pre-polymers or pre-compounds and thenconverted, such as epoxy-resins, polyurethanes, phenol-formaldehyderesins, and melamine-formaldehyde resins. Many display and electricalapplications may prefer acrylic based polymers, silicones and suchstructural aiding layers, for example, commercially availableSentryGlas® from BuPont. The polymer layers may be transparent for someapplications, but need not be for other applications.

Non-limiting examples of adhesive materials for laminating the polymerlayers to the flexible glass substrates at elevated temperatures includeUV curable optical adhesives or optical cements such as thosemanufactured by Norland™ Optical Adhesives (NOA60, NOA61, NOA63, NOA65,NOA68, NOA68T, NOA71, NOA72, NOA73, NOA74, NOA75, NOA76, NOA78, NOA81,NOA84, NOA88, NOA89), Dow Corning™ (Sylgard 184 and other thermallycuring silicones), Dymax™, and others. For heat-activated adhesivematerials (e.g., NOA83H), adhesive materials with activationtemperatures of greater than a preselected temperature (e.g., about 50°C. or more, such as about 70° C. or more, such as 80° C. or more, suchas 100° C. or more) may be used to allow the polymer layer anopportunity to expand relative to the flexible glass substrate prior toits lamination thereto.

Additionally, each of the polymer layers may itself be a laminated orcomposite structure made of different types of polymer having differentYoung's moduli, different Poisson's Ratios, and/or layer thicknesses. Inthis case, one of skill in the art would be able to homogenize thecompound layer to find effective values for the overall layer, includingan effective thickness, an effective Young's modulus, and an effectivePoisson's Ratio that may be used as described herein to beneficiallyconfigure a glass-polymer laminate. The composites, for example, may beformed of any combinations of the above materials and/or metals, such asstainless steel, nickel, copper, noble metals, metal oxides, etc.

The glass-polymer laminates described herein may be used as a substratefor mounting device-functional layers, or may be used as an encapsulantlayer or barrier layer within the device. The device may be anelectronic device, for example, a display screen (including a LiquidCrystal Display, a Plasma Display, an Organic Light Emitting Diodedisplay, flat panel display, for example), a lighting-emitting device,or a solar cell module. The functional layers may include, for example,thin film transistors (TFTs), diodes, photodiodes, triodes, photovoltaiccells, photocouplers, transparent electrodes, color filter, or anelectroconductive layer. The glass-polymer laminate may be used as acover laminated onto the display screens. The glass-polymer laminate maybe used as a substrate/encapsulant not only for OLEDs (small moleculefluorescence (SMF) and (LEP) light emitting polymers) but for otherdevices including an electrically active layer e.g. organicphoto-detectors, organic solar-cells, thin-film-transistor (TFT) arraysand TFTs for OLEDs. Another use is for LEP products such as un-patternedbacklights and other light sources or patterned devices such as signs,alpha-numeric displays or dot-matrix and other high-resolution displays.

The glass-polymer laminate may be a substantially transparent formableand/or flexible structure for use as a protective element in anelectronic device, wherein the glass-polymer laminate is a compositestructure comprising a layer of glass of a thickness from 5 to 300microns, and a layer of polymer ranging in thickness from 50 microns to1 cm or more. In this connection, the formability of the glass-polymerlaminate allows it to deviate from full planarity by bending and/ortwisting so it can adapt to the shape or form of some other object. Itsflexibility allows it to be bent without detrimentally affecting itsbarrier properties.

The glass-polymer laminate can constitute a substrate for an electronicdevice and, as such, can be coated with a transparent electrode layer.The layer may be the anode and may be indium tin oxide or silver basedconductors. As alternatives, the glass-polymer laminate may constitutean encapsulation film for light-emitting, or other electronic device.

The electronic device with the glass-polymer laminate can bemanufactured in a sequence of integrated steps which include theconstruction of the glass-polymer laminate, deposition of thetransparent electrode layer, deposition of the or each electricallyactive layer and deposition of the second electrode layer. A batch,semi-continuous or continuous process can be considered for themanufacture of the complete device. A further encapsulation layer on thesecond electrode layer can be provided. Various techniques formanufacturing the glass-polymer laminate are possible in accordance withdifferent embodiments.

According to one embodiment, a polymer layer carrying a coating of afirst transparent electrode (e.g. ITO) is provided. Then, at least onelayer of an electrically active, e.g. electroluminescent, organicmaterial is deposited followed by the second electrode layer. Thecomplete structure is then laminated to the glass layer. According toanother embodiment, the polymer and glass layers are exchanged in thepreceding sequence. According to a further embodiment, the glass-polymerlaminate is prefabricated and is then used as the basis for depositionof the first electrode layer, the at least one layer of an electricallyactive material and the second electrode layer.

The glass and polymer layers can be provided in sheet form according toa batch process. Alternatively, the glass layer can be provided in sheetform and the polymer layer from a continuous roll. As a furtherpossibility, both glass and polymer layers are from continuous rolls.The composite structure can be formed by lamination of the glass andpolymer layers, e.g. according to a batch process, a continuousroll-to-roll process or a semi-continuous process whereby the polymerlayer is a continuous film and the glass layer is in sheet form.

For the polymer layer, it is possible to use polymers which can bedeposited/coated as pre-polymers or pre-compounds and then converted,such as epoxy-resins, polyurethanes, phenol-formaldehyde resins, andmelamine-formaldehyde resins. The lamination of the glass and polymerlayers can be with glue/adhesive in between the layers. In that case,adhesive can be pre-coated onto one of the two or on both substrates; orsupplied during the lamination process, at room or elevated temperatureand with or without pressure. UV-cured glues are also suitable. Thepolymer layer can be in the form of polymer sheets which are pre-coatedwith a heat-seal glue. Lamination and/or deposition of the polymer layeronto the glass layer can be integrated in the fabrication process of theglass, i.e. glass comes off the fabrication line and is then (still hotor warm or cold) coated with the polymer.

As an alternative to formation by lamination, the polymer layer of thecomposite is coated onto the glass layer by a batch or continuousprocess. Coating of the polymer onto the glass can be by dip, spray,solution-spin, solution-blade, meniscus coating, or by coating of amolten polymer onto the glass layer. That is, it is possible to considerthe different situations (i) where polymer exists already as film and islaminated to the glass and (ii) where polymer is not in film form but iscoated onto the glass by dip, spray, etc. etc. The pre-polymersmentioned above, for example, are amenable to case (ii). However, alsoseveral of the other polymers mentioned above can be coated for case(ii). In this instance the polymers can be coated onto the glassprincipally by: coating from solution, from a melt or as pre-polymer.

In manufacture of an electronic device, it is usually necessary tosubject some or all of the layers to processing steps. For example, ifthere is present an electroluminescent organic material that is asemiconductive conjugated polymer such as poly(phenylene vinylene) (PPV)then the deposition of that layer would normally take place bydepositing a precursor to the polymer in a solvent, for example byspin-coating, and then subjecting that layer to a subsequent processingstep to convert the precursor to the final polymer. Thus, the underlyingglass-polymer laminate, if present during these processing steps, mustbe able to withstand the solvents used for spin-coating the precursorlayer and the subsequent temperatures used for driving off the solventand converting the precursor to the polymer. Thus, the polymer layer ofthe glass-polymer laminate needs to be of appropriate qualities. Forexample, if the glass-polymer laminate is to be subjected to hightemperatures, then the glass-transition temperature of the polymer layer(and the working temperature of any adhesive used) should be above thosetemperatures. For example, a temperature of in excess of 150° C. ispossible. Moreover, in certain situations, the polymer layer should beresistant to the solvent layers used for the polymers, such as mixedxylene, THF, used for soluble conjugated polymers such as MEH PPV.

The glass-polymer laminate can comprise more than two or three layers.Referring to FIG. 7, a flexible glass-polymer laminate structure 120includes more than three layers, in this case, seven layers 122, 124,126, 128, 130, 132 and 134 with layers 122 and 134 labeled Glass 1,layers 126 and 130 labeled Glass 2, layers 124 and 132 labeled Polymer 2and layer 128 labeled Polymer 1. Here, the glass layers 122 and 134 formthe outermost layers. Glass 1 and Glass 2 may have the same or differentglass compositions (including Eg, Vg, α, t_(g)) and may each be a singlehomogeneous sheet, or be glass laminates. Polymer 1 and Polymer 2 may bethe same or of different materials (including Ep, Vp, α_(p), t_(p)) andcan be either a single homogeneous polymer sheet or be laminates ofdifferent polymers.

Referring to FIG. 8, another flexible glass-polymer laminate structure140 includes seven layers 142, 144, 146, 148, 150, 152 and 154 withlayers 142 and 154 labeled Polymer 2, layers 146 and 150 labeled Polymer1, layers 144 and 152 labeled Glass 2 and layer 148 labeled Glass 1.Here, the polymer layers 142 and 154 form the outermost layers (havingEp2, Vp2, α_(p2), t_(p2)/2). Glass 1 and Glass 2 (including: Eg1, Vg1,α_(g1), t_(g1); and E_(g2), V_(g2), α_(g2), t_(g2)/2, respectively) mayhave the same or different glass compositions and may each be a singlehomogeneous sheet, or be glass laminates. Polymer 1 and Polymer 2(including: Ep1, Vp1, α_(p1), t_(p1)/2; and Ep2, Vp2, α_(p2), t_(p2)/2,respectively) may be the same or of different materials and can beeither a single homogeneous polymer sheet or be laminates of differentpolymers.

The above-described flexible glass-polymer laminate structures provideincreased strength to ultra-thin flexible glass substrates. Nearlyconstant uniform compressive stress can be provided through the glassthickness for symmetric laminate structures. The polymer layers canprovide breakage protection and hold the flexible glass substratestogether in the event of any breakage. The flexible glass-polymerlaminate structures can provide touch and cover glass, which could beused to replace chemically strengthened glass. Curved display glass,such as that discussed above in connection with asymmetric flexibleglass-polymer laminate structures can be provided. The flexibleglass-polymer laminate structures can provide a barrier layer for thinfilm PV, such as BIPV applications and provide improved impactprotection for PV modules. The flexible glass substrates can also act asa moisture barrier and block undesired UV light. A potential applicationis as an encapsulant for OLEDs.

Additional functionality can be incorporated into polymer layers. Forexample, the polymer layer can comprise a polymer polarizer sheet, acontrast-enhancing filter-laminate, have anti-reflective properties,color filter properties or color conversion properties. For example, itwould be possible to have a device in which the light emitting layeremits blue light and in which the laminate contains, for example, red orgreen fluorescent molecules which absorb the blue and re-emit in the redor green. Alternatively or additionally, the polymer layer can bedesigned to block undesired ambient light and/or have scatteringparticles so that wave guiding is reduced and the brightness of thedevice is increased. Such additional functionalities could beincorporated in the glass layer. Where a third polymer layer is providedin the composite structure, this allows the possibility of two differenttypes of polymer layers, providing the possibility for incorporatingdifferent additional functionalities into the different layers.

In addition to electronic devices, the above-described flexibleglass-polymer laminate structures may be used in other areas, such asarchitectural surface decoration, protective coatings, electrochromaticwindows, fire resistant surfaces and in various configurations ofmulti-stack structures required to meet ballistic glazing requirements.Similarly, the flexible glass-polymer laminate structures could act as abarrier material to protect films, structures and/or devices from oxygenand moisture ingress/permeation for applications such as organic/thinfilm, PV, OLED display and lighting.

The flexible glass-polymer laminate structures can take advantage ofattributes from two classes of material (organic and inorganic). Polymermaterials are easily scratched, degrade from environmental elementsincluding sunlight exposure and provide poor moisture/oxygen barrierproperties. Glass, on the other hand, is scratch resistant, durable andis known for excellent moisture/oxygen barrier properties. However,glass has higher density compared to polymer and is a brittle materialwhere strength of glass is dictated by defects and flaws. The abovedescribed flexible glass-polymer laminate structures and methods ofmaking them take advantage of these two classes of materials andcombining into one laminate structure having improved barrierproperties, lightweight and higher mechanical reliability compared to abare thin glass stack.

CONCLUSION

It should be emphasized that the above-described embodiments of thepresent invention, particularly any “preferred” embodiments, are merelypossible examples of implementations, merely set forth for a clearunderstanding of various principles of the invention. Many variationsand modifications may be made to the above-described embodiments of theinvention without departing substantially from the spirit and variousprinciples of the invention. All such modifications and variations areintended to be included herein within the scope of this disclosure andthe present invention and protected by the following claims.

What is claimed is:
 1. A method of forming a flexible glass-polymerlaminate structure, the method comprising: heating a polymer layer ofthe flexible glass-polymer laminate structure to an elevated temperatureof greater than 20° C., the polymer layer having a coefficient ofthermal expansion (CTE) that is at least 2 times a CTE of a flexibleglass substrate of the flexible glass-polymer laminate structure, orhaving a coefficient of thermal expansion (CTE) that is at least about 3ppm/° C. greater than a CTE of the flexible glass substrate of theflexible glass-polymer laminate structure; laminating the polymer layerat the elevated temperature to the flexible glass substrate; and coolingthe polymer layer below the elevated temperature to introduce acompressive stress of at least about 30 MPa across a thickness of theflexible glass substrate.
 2. The method of claim 1, wherein the flexibleglass substrate has a compressive stress of at least about 80 MPa acrossa thickness of the flexible glass substrate.
 3. The method of claim 1,wherein the polymer layer has a CTE that is at least about 10 times theCTE of the flexible glass substrate.
 4. The method of claim 1,comprising expanding the polymer layer relative to the flexible glasssubstrate as the polymer layer is heated to the elevated temperature. 5.The method of claim 1, further comprising applying an adhesive layerbetween the flexible glass substrate and the polymer layer thatlaminates the polymer layer to the flexible glass substrate.
 6. Themethod of claim 1, further comprising cooling the polymer layer belowthe elevated temperature to bend the flexible glass substrate whilesimultaneously providing a compressive stress across at least a portionof thickness of the flexible glass substrate.